gnu: bootloader: Deprecate "device" field in favor of "target".

* gnu/bootloader.scm (<bootloader-configuration>): Deprecate "device" field in
favor of "target" field.  This is mostly a renaming but also a generalization
to support UEFI targets being paths to a mounted partition instead of a device
name.
* gnu/system/examples/bare-bones.tmpl:
* gnu/system/examples/desktop.tmpl:
* gnu/system/examples/lightweight-desktop.tmpl:
* gnu/system/examples/vm-image.tmpl:
* gnu/system/install.scm:
* gnu/tests.scm:
* gnu/tests/install.scm:
* gnu/tests/nfs.scm:
* tests/system.scm: Adapt all invocations of bootloader-configuration.
* guix/scripts/system.scm (perform-action): Rename device argument to
bootloader-target.
(process-action): Adapt caller.
* doc/guix.texi (Proceeding with the Installation):
* doc/guix.texi (Bootloader Configuration): Update documentation.